There is a continuous trend of scaling transistor sizes in the development of integrated circuit technology, which, however, brings on the well known short channel effects. In recent years, an ultra-thin SOI transistor is proposed, which has a fully-depleted channel region in a top semiconductor layer of an ultra-thin SOI substrate and short channel effects in the transistor are suppressed well.
As an example, Cheng et al. proposed an ultra-thin SOI transistor structure in “Fully Depleted Extremely Thin SOI Technology Fabricated by a Novel Integration Scheme Featuring Implant-Free, Zero-Silicon-Loss, and Faceted Raised Source/Drain”, 2009 Symposium on VLSI Technology Digest of Technical Papers, p 212-213.
As shown in FIG. 1, a conventional ultra-thin SOI transistor is formed on an SOI substrate including a base substrate 11, a buried oxide layer (BOX) 12, and a top semiconductor layer 13. The transistor comprises a channel region formed in the top semiconductor layer, a gate stack including a gate dielectric 14 and a gate conductor 15 formed above the channel region, a sidewall spacer 16 surrounding the gate, and RSDs 17a, 17b. 
In such an ultra-thin SOI transistor, the RSDs can reduce the resistance of source/drain region and minimize parasitic capacitances of gate-source and gate-drain. Moreover, when silicides are formed at the top of the source/drain regions, the RSDs ensure that a sufficient amount of Si is involved in the silicidation and avoid Si in the source/drain regions from being exhausted in the silicidation.
However, the formation of RSDs needs to pre-clean the top semiconductor layer of the ultra-thin SOI substrate and epitaxially grow a silicon layer thereon after formation of the gate and sidewall spacers surrounding the gate. This increases the fabrication complexity. Moreover, the portion of the top semiconductor layer of the ultra-thin SOI substrate below the sidewall spacer has a high resistance and causes a very high ON-state resistance since this portion is a part of the source-drain conductive path.